This invention relates to implantable bone prostheses, and more particularly to knee joint prostheses.
Joint replacement surgery is quite common and enables many individuals to function normally when otherwise it would not be possible to do so. Artificial joints are normally composed of metallic, ceramic and/or plastic components that are fixed to existing bone.
Knee arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural knee joint is replaced with a prosthetic knee joint. Typical knee prostheses include a femoral component, a patella component, a tibial tray or plateau, and a tibial bearing member. The femoral component generally includes a pair of laterally spaced apart condylar portions, the distal surfaces of which articulate with complementary condylar elements formed in a tibial bearing component.
In a properly functioning artificial knee joint, the condylar portions of the femoral component must slide and roll freely over the articulation surface formed by the condylar elements of the tibial bearing member. Natural friction within a replaced, artificial joint can lead to the development of wear debris in which minute particles of debris (e.g., metal or plastic from the prosthesis) become dislodged and migrate within the joint. The phenomenon of wear debris within artificial joints is a serious problem that can inhibit the proper mechanical functioning of the joint. Moreover, wear debris can lead to osteolysis and bone deterioration. When wear debris develops within an artificial joint, surgical removal of the debris or subsequent replacement of the artificial joint is often necessary.
During normal usage of a properly implanted prosthetic knee joint, load and stress are placed on the tibial bearing member. The tibial bearing member is typically made of an ultrahigh molecular weight polyethylene (UHMWPE). Friction, continuous cycling and stress can cause some erosion and/or fracture of the tibial bearing member, thus leading to wear debris. The risk of wear debris can be even greater during malalignment of an artificial knee joint, which can result from normal usage or from imperfect and/or inaccurate implantation of the prosthesis within a patient. As a result of malalignment, the load upon the tibial bearing member is not evenly distributed. Instead, excess load is placed on certain areas of the tibial bearing member. This uneven distribution of load (or edge loading) can accelerate the development of wear debris. Contact stresses on the tibial bearing member increase substantially with malalignment of the joint, thus increasing the risk that wear debris will develop when a prosthetic knee joint is subjected to malalignment conditions.
Contact stresses on the tibial bearing member also tend to increase when the prosthetic knee joint is rotated into flexion. This increased contact stress results from a corresponding decrease in tibio-femoral contact area.
There is thus a need for knee joint prostheses that have reduced tendency to develop wear debris due to the maintenance of good contact area and low contact stress between femoral and tibial components, even during the dynamics of daily activity and in various conditions of flexion and malalignment.
Accordingly, it is an object of the present invention to provide knee joint prostheses with improved performance and a longer use for life. It is also an object of the invention to provide knee joint prostheses having reduced tendency to develop wear debris. A further object of the invention is to provide knee joint prostheses which are able to maintain relatively high contact area and low contact stress between femoral and tibial components throughout the normal range of motion and in conditions at malalignment. Another object of the invention is to provide knee joint prostheses that exhibit acceptable levels of laxity despite maintaining good tibio-femoral contact area in conditions of flexion. These and other objects will be apparent from the description that follows.